Metropoulos Forum

So back in June I decided to build an amp that I'd cherish and never let go off, and I went with the so-called Holy Grail of Marshall, the JTM45/100 Supa amp. And now here it is, freshly finished just an hour ago, passed all the tests, running fine and high. The tone is so sweet and it's not even started to bed in yet. Many thanks to Larry, Toner, axeman, Billy Batz, specially Neil, and many others, for helping me with almost every step. Now for your viewing pleasure (I always got problem posting pics here, if someone know how to please help me):

http://s64.photobucket.com/albums/h174/ ... 0%20Build/" onclick="window.open(this.href);return false;

I populated the boards while waiting for the other shipments. The chassis got here on Friday and I spend the whole weekend to finish the build. I tried to slow down but I can't even sleep well knowing it's there waiting to be built. I don't have a wire stripper and that slowed me down quite a bit though.

Rushing the work has taken its toll however. I made 2 mistakes that I know of. The first was that I cut one leg of the 0.1uF on Presence pot too short. I fixed that by soldering a short buss wire to the leg and then to the pot. The 2nd mistake was that I soldered the purple wire from the 27K resistor to the Bass pot. The wire was cut too short to reroute to the Presence pot. So I added a buss wire there, tubed it and heat shrinked the joint. My question is is there any harm in fixing those errors that way? Could add a little noise at least I guess?

Btw I fired the amp up for the first time at midnight. Thanks to the PPIMV I was still be able to play for a good hour. Can't wait till tomorrow to play it more

I've got some more problems though. The maximum voltage on pin 5's of power tubes is -50VDC, lowest is -70VDC. When I did the pin 5 voltage test like in the Metro instruction I left it at -50VDC and believed that was the lowest (Metro plexi instruction specified the range being -30 to -50VDC). Turned out the bias reached 75mVDC . I guess I was extremely luck as nothing went wrong when I installed the power tubes and fired it up for the first time. It then was weird because with that extreme 75mVDC bias voltage, the bias pot was fully counter clockwise that I thought would have lowest bias mV. I had turn it clockwise to reduce the bias volt to 38mVDC (is this a desirable value by the way?). Can someone clear this up for me? Is the pin 5 reading normal?

Looks good.

You are going to need to play with your bias resistor value (sorry clicked away from pics) to get more into the - 45 to - 50 range on that pin.

Are you using the 490v taps?

If so 36mA is the target. For 540v taps 33mA is about right:

Correct bias in mA = 17.5/pin 3 voltage x1000

neikeel wrote:Looks good.

You are going to need to play with your bias resistor value (sorry clicked away from pics) to get more into the - 45 to - 50 range on that pin.

Are you using the 490v taps?

If so 36mA is the target. For 540v taps 33mA is about right:

Correct bias in mA = 17.5/pin 3 voltage x1000

Thanks Neil. Yea I'm using the 490V taps. What is that bias resistor though? Is that important that I get into the right range (-45 to -50V)?

Congratulations on your build!

I can't see the photos, but if you have a bias pot wired in like this: http://metroamp.com/wiki/index.php/Installing_Bias_Pot You can change the value of the resistor soldered to the pot leg to get the correct range.

AJW wrote:Congratulations on your build!

I can't see the photos, but if you have a bias pot wired in like this: http://metroamp.com/wiki/index.php/Installing_Bias_Pot You can change the value of the resistor soldered to the pot leg to get the correct range.

Thanks buddy!

That looks exactly what I did for the bias pot. And again is it important that I get into the correct range? My bias sweep is now around 25mV to 75mV

That range is fine.

AJW wrote:That range is fine.

Cool, thanks!

One more !! Congrats ! yeah, i guess how happy you can be now.
send some clips some day !

Xplorer wrote:One more !! Congrats ! yeah, i guess how happy you can be now.
send some clips some day !

Thanks! I'm so proud of it . Sometimes I enjoyed just staring at it. And it looks really cool without the plexi panels. I used a RC Booster and played a little Hendrix, Lynyrd Skynyrd and Van Halen today and couldn't believe what I was hearing. This is THE tone and it is far superior to my playing . I just can't imagine how it'll sound after bedding in a fair amount. This amp has some serious mojo. I think I'll have to give it a name

The Lar/Mar MV is a very good mod by the way. I also find running effects out in front of the amp is not as horrible as I thought it would be. I've only got delay, chorus and reverb anyway. It's just so good having a simple one channel amp and a few effects. I can understand now why a lot of folks keeps coming back to Marshall after their long expensive journey to find the sounds in their heads. Whenever financially possible, I'll be building a '67 Superbass.

I just checked out the photos, and the build looks great!

AJW wrote:I just checked out the photos, and the build looks great!

Thanks man, so does yours. Your pics actually helped me quite a bit during the process

I'm glad they helped. I was using a picture of George's 45/100 as a guide myself. It was especially helpful when I was making my own boards. There is a drawing of the power supply board posted somewhere in this section that is a little off. I actually built a board to that, but it doesn't work quite right. I built a second board using the Metropoulos 45/100 as a guide, and it was perfect.